I was doing more testing when you posted about the bug report... I added this comment on what I found:
Further testing I did showed that on the first tick of the game (tick 0), each guard got the correct target waypoint to the next waypoint (since they start on a waypoint). Tick 1 they still have the correct GridTargetX and Y, on Tick 2 the GridTargetX is changed (even though no actions were executed). They all arrive at the "new" target coordinates and stop - just not on the next intended waypoint, so they do not get further instructions.
For Guard Id 1, his GridTargetX was changed from 41 to 40. He then stops at that spot, 1 grid step away from the next waypoint.